The long-term goal of this project is to elucidate the molecular regulation of photoreceptor production in the vertebrate retina. This knowledge is imperative to the development of effective stem cell-based photoreceptor replacement therapies. Much needs to be learned about the identities of the genes involved and how they contribute to the selection of the photoreceptor fate from among the other options. Studies during the previous funding period identified neuroD as an instrumental player in photoreceptor differentiation, in a transcriptional pathway of ngn2->neuroD->RaxL. In this pathway, ngn2 functions in multipotent progenitors and its downstream genetic targets include neuroD;neuroD elicits a photoreceptor development program, including the expression of homeobox gene RaxL. In this application we will test the hypothesis that photoreceptor production employs ngn1 at a key step between ngn2 and neuroD. Aim 1 examines whether ngn1 is expressed at the right place and the right time to be a major gene in leading progenitors to the photoreceptor path. Aim 2 investigates whether ngn1 expressly steers progenitors to the photoreceptor path and, thus, leads exclusively to photoreceptor production. Aim 3 determines whether ngn1 is required for photoreceptor production. Aim 4 addresses how ngn1 genetically relates to ngn2 and neuroD during photoreceptor production. These studies will be carried out with a battery of techniques from molecular biology, cell biology, developmental biology, and genetics. This project promises to shed light on the transcriptional regulation governing photoreceptor production. Furthermore, it bears clinical implications. The identification of key genetic players in photoreceptor production will capacitate efficient in vitro or in vivo photoreceptor generation for studies with therapeutic goals. These studies are timely in this era of heightened interest in stem cell research for replacement therapies.